X-ray tubes, gas-filled X-ray detectors and various other applications require window materials with which it is possible to seal an opening in a gastight manner, while still letting X-rays of at least some desired wavelength range pass through the window with as little attenuation as possible. Another requirement for the window material is its ability to stand a certain amount of mechanical stress, because the pressure difference between the different sides of the window may be considerable.
In this description we use the term “film” to mean a thin material layer of uniform thickness, and the term “membrane” to mean generally a structure that is relatively thin, i.e. has a very small overall dimension in one direction compared to its dimensions in the other, perpendicular dimensions. A membrane may consist of several materials and may have significant local variations in its thickness, and may exhibit structural topology, such as reinforcement ridges.
A prior art membrane structure for radiation windows is known from the patent publication GB 2 266 787 A. FIG. 1 illustrates the corresponding manufacturing method, although schematically only and not to scale. A thin polyimide film 101 is first manufactured on the surface of a substrate 102. The thickness of the polyimide film 101 may be e.g. in the order of 300 nanometers. A thick layer 103 of a photoresist is spread onto the polyimide film 101, and exposed through an exposure mask 104. Here we assume that the photoresist layer 103 consists of a positive resist, which means that the exposed portions will be dissolved, leaving a reinforcement grid 105 attached to the thin polyimide film 101. A negative resist could be used as well, but since the aim is to produce the grid 105, using a negative resist would necessitate forming the exposure mask 104 differently so that it would allow the beam parts of the grid to be exposed instead of the opening parts like in FIG. 1. Finally the substrate is etched away, possibly leaving an attachment ring 106 made of the substrate material at the edges. The thickness of the reinforcement grid 105 can be several micrometers, or even 15 micrometers.
Polyimide is not very gastight by itself, so a so-called diffusion barrier layer 107 is usually made on the even surface of the thin polyimide film 101. Known materials for the diffusion barrier layer 107 include but are not limited to ceramic materials and diamond. These have very advantageous properties in terms of producing a very gastight membrane, but they are also very hard and brittle. A more ductile diffusion barrier layer can be made by first depositing a thin layer (like 40 nanometers) of aluminum nitride 108 onto the even surface of the thin polyimide film 101 and covering it with a layer of pure aluminum 109.
Prior art window membranes like that shown in FIG. 1 exhibit certain disadvantageous properties. Selecting the thicknesses of the polyimide film 101 and the diffusion barrier layer 107 is an awkward tradeoff: the thicker the layers, the better tightness can be achieved, however with the associated increase in X-ray attenuation. When an optimally thin diffusion barrier layer is aimed at, it may happen that a small pinhole is left in the diffusion barrier layer materials, or a grain boundary is created through which gas molecules can penetrate the diffusion barrier layer, causing leakage.